Presently, various plastic containers are used for packaging because of their advantages such as light weight, transparency and easiness of molding.
However, because an oxygen barrier property of the plastic containers is lower than those of metal containers and glass containers, the plastic containers have problems in that the contents of the containers deteriorate due to chemical oxidation and the action of aerobic bacteria.
For preventing these problems, the plastic container walls have a multi-layer structure in which at least one layer is made of a resin having an excellent oxygen barrier property such as ethylene-vinyl alcohol copolymer. In addition, there are other kinds of containers having an oxygen-absorbing layer for removing oxygen remaining in the containers and also oxygen penetrating into the container from the outside. Oxygen absorbers (deoxidizers) used for the oxygen-absorbing layer include, for example, those mainly containing a reducing substance such as iron powder (see, Japanese Examined Patent Publication (JP KOKOKU) No. Sho 62-1824).
A method in which an oxygen absorber such as iron powder is incorporated into a resin, and the resulting resin composition is used as a material for the wall of a container used as a packaging material shows a sufficiently high ability to absorb oxygen, but the resulting resin composition has a color hue peculiar to the iron powder. Therefore, said method is limited in its application and cannot be used in the field of packaging in which transparency is required.
Further, there have been disclosed, as a resin-based oxygen-absorbing material, an oxygen-absorbing resin composition comprising a resin having a carbon-carbon unsaturated bond and a transition metal catalyst (see, Japanese Un-Examined Patent Publication (JP KOKAI) No. 2001-39475, Japanese Un-Examined Patent Publication (JP KOHYO) No. Hei 8-502306 and Japanese Patent No. 3,183,704) and an oxygen-absorbing resin composition comprising a resin having a cyclic olefin (cyclohexene) structure and a transition metal catalyst (in particular, a Co salt) (see, Japanese Un-Examined Patent Publication (JP KOHYO) No. 2003-521552 and Japanese Un-Examined Patent Publication (JP KOKAI) No. 2003-253131). However, the former composition has a problem in that the molecular chain of the resin is cleaved as the resin absorbs oxygen and thus low molecular weight organic components are generated as an odor component. On the other hand, the latter composition comprises ring structures as the oxygen-absorbing sites, and thus it could somewhat inhibit the generation of such low molecular weight organic (odor) components, but there is a tendency that the use of such a transition metal catalyst (a Co salt), and further, an irradiation treatment with ultraviolet rays or the like for activating reactions, may easily result in the occurrence of reactions at sites other than the expected oxygen-absorbing sites and this in turn leads to the formation of decomposition components.
The present inventors have developed an oxygen-absorbing resin composition comprising a polyester manufactured by using as a raw material a mixture of isomers of derivatives (including alkyl substitution compounds) of tetrahydrophthalic anhydride having, at its Δ4 position, a carbon-carbon double bond (see, the pamphlet of International Patent Publication No. 2005/105887). However, when these resin compositions are to be used for oxygen-absorbing containers, an irradiation treatment or the like is necessary.